Silver powder and method for producing same



rates SILVER POWDER AND NETHOD FGR PRODUCING SAME No Drawing. Application June 13., 1955, Serial No. 515,239

Claims. (Cl. 75-=11$) This invention relates to silver powders and more particularly it relates to precipitated silver powders having a high apparent density.

Precipitated silver powder is customarily prepared by the following process:

Metallic silver is dissolved in aqueous nitric acid having a concentration between about 30% and 80% HNOa (about 225 B to 41.5 B) to obtain a silver nitrate solution containing between about 550 to 940 grams per liter of silver, calculated as metallic silver, and containing from about 1 to 35 grams per liter of residual HNOs. Sodium carbonate is added in excess to the silver nitrate solution to neutralize all the acid and precipitate all the silver as silver carbonate. In order that all silver be recovered it has been customary to add the sodium carbonate in considerable excess, for example, 50% to 100% excess over that required to neutralize the acid. The silver carbonate slurry containing the excess carbonate was then reduced to metallic silver powder by the addition thereto of a reducing agent, for example, glycerine or aqueous formaldehyde. The precipitated silver powder heretofore produced in this manner had an apparent density of between about 0.7 to 1.5 grams per cc.

Many of the specific uses of silver powders require the use of such powdered silvers having a high apparent density. For example, silver printing pastes for use in the production of electronic components require a high electrical conductivity and, therefore, necessitate the use of a silver paste having a high proportion of silver particles. Such silver pastes can only be prepared by the use of silver powders having a high apparent density. Attempts have been made to increase the apparent density of precipitated silver powder by such methods as grinding or milling whereby the treated powder will pack more densely and produce a silver having a higher apparent density. Such processes are not only expensive but involve a certain degree of contamination of the silver powder.

Moreover, the different uses of silver powders require silver powders of varying degrees of apparent density and the art is in great need of an adequate process for producing precipitated silver powder of any desired given apparent density between 1.5 and 5.0 grams per cc.

The apparent density of silver powders is generally measured by placing 50 to 100 grams of the silver powder into a lOtl-cc. cylinder and tapping the cylinder until maximum settling takes place. The volume occupied by the powder is measured and the density in grams per cc. calculated. This is a standard control test for silver powders.

It is an object of this invention to produce precipitated silver powder having an apparent density of between 2 and 5 grams per cc.

It is another object of this invention to provide a process for the production of precipitated silver powder having any desired apparent density ranging from 1.5 to 5.0 grams per cc.

It is still another object of this invention to produce aqueous compositions of silver comprised of precipitated 2,?5Z,237 Patented .Fune 26, 1956 silver powder having an apparent density of between 2 and 5 grams per cc., which compositions will not gel on the addition thereto of silver nitrate.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in general, by adding to a silver nitrate solution, containing about 550 to 940 grams per liter of silver calculated as silver metal and containing not to exceed about 35 grams per liter of residual nitric acid, a suflicient volume of sodium carbonate to precipitate all of the silver and react with all of the nitric acid but with a residual concentration of sodium carbonate of less than 25% of the Weight of the silver content of the silver nitrate. It has been found in accordance with this invention that the apparent density of the silver powder constituting the final product can be readily controlled to any value between 1.5 and 5.0 grams per cc. by controlling the amount ofthe sodium carbonate added to the silver nitrate solution. This excess of sodium carbonate must be a quantity between 0.5% and 25 of the weight of silver content of the silver nitrate slurry. The smaller the excess quan tity of sodium carbonate added, the higher will be the apparent density of the silver powder constituting the final product.

In carrying out the process of this invention it is prefcrred, but by no means essential, to start with a silver nitrate solution containing between 600 and 700 grams per liter of silver (Ag), and not to exceed 10 grams per liter residual HNOs.

The sodium carbonate added to the silver nitrate solution may be in the form of a dry powder or as a concentrated aqueous solution. The excess sodium carbonate may be added, if desired, in several increments. A first increment may be added to precipitate all of the silver and react with all of the nitric acid and the excess sodium carbonate may be added as a second increment. In this manner, the excess sodium carbonate can'be easily controlled. The entire amount of sodium carbonate, i. e., the amount necessary for neutralization of the silver nitrate and the desired excess may be added as a single step.

In place of sodium carbonate, potassium carbonate or sodium or potassium bicarbonate or sodium or potassium hydroxide may be used both as a neutralization agent or excess, based on their alkaline equivalent to the amounts of sodium carbonate above specified.

The reduction of the silver carbonate may be accomplished in the usual manner with a suitable reducing agent, preferably an aqueous solution of formaldehyde. The formaldehyde solution is added to the silver carbonate slurry until the reaction mass first turns black and then to the dark grey color of powdered silver.

The formaldehyde added for reduction may be in the form of a 10% to aqueous solution and is preferably added in excess of the amount required to reduce all of the silver carbonate to powdered silver. During this reduction, the pH decreases, possibly requiring further addition of a small amount of soda ash near the end of the run to recover all of the silver.

After reduction of the silver carbonate to silver powder, the powder is filtered from the slurry, washed with deionized water, again filtered, and dried by heating out of contact with the atmosphere.

The following examples are given to illustrate the manner in which the final density of the silver powders may be controlled by the addition of excess soda ash in the manner above described.

In the following examples, the designated pounds of excess soda ash are added as the second increment to 1500 troy ounce batches (46,650 grams) of silver dissolved in commercial nitric acid 42 B and neutralized with a first increment of soda ash but containing less than 5 grams Example I Density obtained 1.5 to 2.0 Pounds excess soda ash added 18 Grams excess soda ash added 8160 Per cent of silver weight percent 17.5 Range do 12 to 25 Example II Density obtained 2.0 to 3.0

Pounds excess soda ash added 11.5 Grams excess soda ash added 5000 Per cent of silver weight "percent" 10.7 Range do 8 to 12 Example 111 Density obtained 3.0 to 4.0 Pounds excess soda ash added 5.0 Grams excess soda ash added 2270 Per cent of silver weight percent 4.9 Range do 3 to 8 Example IV Density obtained 4.0 to 5.0 Pounds excess soda ash added 1.5 Grams excess soda ash added 680 Per cent of silver weight percent 1.4 Range do 0.5 to 3.0

The smaller the amount of soda ash added as excess soda ash to control the apparent density of the final powder, the more rapidly the reaction mixture becomes acid upon reduction with formaldehyde and the more silver will be lost in the filtrate unless a slight addition of soda ash is made at the end of the reduction to recover this silver. This slight addition after reduction does not affect the apparent density of the final silver powder.

It is also possible to carry out the present invention in a continuous process by first neutralizing the silver nitrate with soda ash and a second increment of soda ash simultaneously with the formaldehyde solution to maintain the required alkalinity for high density silver precipitation. The more nearly neutral (or slightly acid) the reaction batch is maintained, the higher will be the apparent density of the silver.

Example V Fifteen hundred (1500) troy ounces of silver are dissolved in concentrated commercial nitric acid to produce a solution of silver nitrate containing 857 grams of silver per liter, and 4.7 grams of residual nitric acid per liter. This solution is allowed to flow into an aqueous solution containing 75 pounds of soda ash of specific gravity 1,220 (264 grams per titer of sodium carbonate). After precipitation of the silver carbonate, 50 pounds of excess soda ash are added as a dry powder. A 37% aqueous formaldehyde solution is allowed to flow into this mixture until reduction is complete. The resulting silver powder has a density of 1.19 grams per cc.

Example VI A run similar to that of Example V is made by using the same amount of silver but a solution having a concentration of 633 grams of silver per liter and 8.4 grams of residual nitric acid per liter with the same amounts of soda ash, producing a silver having a density of 1.2 grams per cc.

Example VII Fifteen hundred (1500) troy ounces of silver are dissolved in concentrated commercial nitric acid to produce a solution containing 600 grams of silver per liter and about 6 grams of residual acid per liter. This solution is allowed to flow into a solution containing 59 pounds of soda ash in 192 pounds of water, followed by an excess of 11.5 pounds of soda ash at the beginning of reduction with a 37% formaldehyde solution. After the reduction is complete, 15.3 pounds of soda ash are added to the mass. After reduction, the resulting silver powder has an apparent density of 2.9 grams per cc.

Example VIII In a run similar to that of Example VII if the excess of soda ash added at the beginning of the reduction is 18 pounds instead of 11.5 pounds, and 15.3 pounds at the end of the reduction, the apparent density of the precipitated silver powder is only about 1.7 grams per cc.

Example IX If the run of Example VII is repeated with no excess sodium carbonate added before the reduction and 15 pounds after the reduction, the apparent density of the precipitated silver powder is 4.5 grams per cc.

By the use of precipitated silver powder having a density of 2.5 to 3.0 grams per cc., it is possible to manufacture silver pastes for screen stencil printing that contain more than 60% silver. With high density silver powder having a density of 4 to 5 grams per cc., silver pastes containing over and up to about silver may be prepared.

Silver pastes for screen stencil printing are customarily prepared by mixing silver powder with a vehicle such as described in Deyrup Patent No. 2,379,507 and containing,

for example, ethyl cellulose, hydrogenated rosin, lecithin,

alkyd resins and volatile solvents such as terpene solvents or petroleum solvent; or as described in Craven Patent No. 2,694,016 and containing Waxes, thermoplastic resins and lecithin or phosphorated tall oil.

The high density precipitated silver powders of the present invention, having a density of 2 to 5 grams per cc., may be used to produce exceptionally desirable water base silver compositions for use in printing electrically conductive silver circuits. Such water base composition containing methyl cellulose as a temporary binder will not gel upon addition of silver nitrate. The addition of silver nitrate is of utility in producing a silver paste or semi-liquid having a high electrical conductivity before drying in the semi-liquid form.

Water base silver compositions for use in printing electrically conductive silver circuits are commonly prepared described in Gray Patent No. 2,695,275 and containing, for example, precipitated silver, ceramic flux polyvinyl alcohol, methyl cellulose, ammonia, and water.

When screen stencil silver pastes are prepared with 60% or more silver powder prepared by Example V or Vl they will be very thick; however, when prepared with silver in accordance with Example VIII they will be thinner, and when similar compositions are prepared from silver in accordance with Example VII they will be quite thin and may require the addition of thickening agents.

Water base silver compositions containing methyl cellulose as a temporary binder prepared with precipitated silver powder made by Examples V, VI, or VIII will gel in the presence of silver nitrate added thereto. Similar compositions using silver powder prepared in accordance with Example VII will be satisfactory for many days or 2. In a process for the preparation of precipitated silver the step which comprises adding to an aqueous silver nitrate solution containing between 550 and 940 grams per liter of silver (calculated as metallic silver) and between 1.0 and 35 grams per liter of the residual nitric acid, such a quantity of an alkali metal salt taken from the group consisting of sodium and potassium carbonates, bicarbonates and hydroxides in excess of that which will precipitate substantially all of the silver from said solution and will react with substantially all of the nitric acid as will constitute the alkaline equivalence of between 0.5 and 25 by weight of said silver content of sodium carbonate.

3. In a process for the preparation of precipitated silver the step which comprises adding to an aqueous silver nitrate solution containing between 600 and 700 grams per liter of silver (calculated as metallic silver) and between 1.0 and 35 grams per liter of the residual nitric acid, such a quantity of an alkali metal salt taken from the group consisting of sodium and potassium carbonates, bicarbonates and hydroxides in excess of that which will precipitate substantially all of the silver from said solution and will react with substantially all of the nitric acid as will constitute the alkaline equivalence of between 0.5% and 25% by weight of said silver content of sodium carbonate.

4. In a process for the preparation of precipitated silver the step which comprises adding to an aqueous silver nitrate solution containing between 550 and 940 grams per liter of silver (calculated as metallic silver) and between 2.0 and grams per liter of the residual nitric acid, such a quantity of an alkali metal salt taken from the group consisting of sodium and potassium carbonates, bicarbonates and hydroxides in excess of that which will precipitate substantially all of the silver from said solution and will react with substantially all of the nitric acid as will constitute the alkaline equivalence of between 0.5% and by weight of said silver content of sodium carbonate.

5. In a process for the preparation of precipitated silver the step which comprises adding to an aqueous silver nitrate solution containing between 600 and 700 grams per liter of silver (calculated as metallic silver) and between 2.0 and 10 grams per liter of the residual nitric acid, such a quantity of an alkali metal salt taken from the group consisting of sodium and potassium carbonates, bicarbonates and hydroxides in excess of that which will precipitate substantially all of the silver from said solution and will react with substantially all or" the nitric acid as will constitute the alkaline equivalence of between 0.5% and 25% by weight of said silver content of sodium carbonate.

6. The method of preparing precipitated silver which comprises adding to a silver nitrate solution containing between 550 and 940 grams per liter of silver and not to exceed grams per liter of residual nitric acid such a quantity of an alkali metal salt taken from the group consisting of sodium and potassium carbonates, bicarbonates and hydroxides as will precipitate substantially all of the silver from said solution and react with all of the nitric acid, then adding thereto such a second quantity of said alkali metal salt as will constitute the alkaline equivalence of between 0.5 and 25% of the weight of the silver content of sodium carbonate whereby to produce silver powder having an apparent density between 1.5 and 5.0 grams per cc. upon reduction of the silver precipitate, the smaller the second quantity of alkali metal salt added the higher the apparent density of the resulting silver powder.

7. The method of preparing precipitated silver which comprises adding to a silver nitrate solution containing between 550 and 940 grams per liter of silver and not to exceed 35 grams per liter of residual nitric acid such a quantity of sodium carbonate in excess of that which will precipitate substantially all of the silver as silver carbonate and react with all of the nitric acid as will constitute between 12% and 25 of the weight of the silver content of the silver carbonate whereby to produce, upon reduction of the silver carbonate to silver, precipitated silver powder having an apparent density between 1.5 and 2.0 grams per cc.

8. The method of preparing precipitated silver which comprises adding to a silver nitrate solution containing between 550 and 940 grams per liter of silver and not to exceed 35 grams per liter of residual nitric acid such a quantity of sodium carbonate in excess of that which will precipitate substantially all of the silver as silver carbonate and react with all of the nitric acid as will constitute between 8% and 12% of the weight of the silver content of the silver carbonate whereby to produce, upon reduction of the silver carbonate to silver, precipitated silver powder having an apparent density between 2 and 3 grams per cc.

9. The method of preparing precipitated silver which comprises adding to a silver nitrate solution containing between 550 and 940 grams per liter of silver and not to exceed 35 grams per liter of residual nitric acid such a quantity of sodium carbonate in excess of that which will precipitate substantially all of the silver as silver carbonate and react with all of the nitric acid as will constitute between 3% and 8% of the weight of the silver content of the silver carbonate whereby to produce, upon reduction of the silver carbonate to silver, precipitated silver powder having an apparent density between 3 and 4 grams per cc.

10. The method of preparing precipitated silver which comprises adding to a silver nitrate solution containing between 550 and 940 grams per liter of silver and not to exceed 35 grams per liter of residual nitric acid such a quantity of sodium carbonate in excess of that which will precipitate substantially all of the silver as silver carbonate and react with all of the nitric acid as will constitute between 0.5% and 3.0% of the weight of the silver content of the silver carbonate whereby to produce, upon reduction of the silver carbonate to silver, precipitated silver powder having an apparent density between 4 and 5 grams per cc.

No references cited. 

1. A PRECIPITATED SILVER POWDER HAVING AN APPARENT DENSITY OF 2 TO 5 GRAMS PER CC.
 2. IN A PROCESS FOR THE PREPARATION OF PRECIPITATED SILVER THE STEP WHICH COMPRISES ADDING TO AN AQUEOUS SILVER NITRATE SOLUTION CONTAINING BETWEEN 550 AND 940 GRAMS PER LITER OF SILVER, (CALCULATED AS METALLIC SILVER) AND BETWEEN 1.0 AND 35 GRAMS PER LITER OF THE RESIDUAL NITRIC ACID, SUCH A QUANTITY OF AN ALKALI METAL SALT TAKEN FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM CARBONATES, BICARBONATES AND HYDROXIDES IN EXCESS OF THAT WHICH WILL PRECIPITATE SUBSTANTIALLY ALL OF THE SILVER FROM SAID SOLUTION AND WILL REACT WITH SUBSTANTIALLY ALL OF THE NITRIC ACID AS WILL CONSTITUTE THE ALKALINE EQUIVALENCE OF BETWEEN 0.5% AND 25% BY WEIGHT OF SAID SILVER CONTENT OF SODIUM CARBONATE. 